Atomizers or spray nozzles are well known in those technologies in which it is desired to disperse or spray a liquid material as a cloud of small droplets. The liquid to be dispersed commonly enters the atomizer under pressure, with the configuration of the atomizer designed to utilize the pressure head as the energy source for causing the dispersal of the liquid.
For viscous liquids the amount of energy required to break the liquid stream into small droplets is so great as to preclude the use of liquid pressure alone as a source. In this situation a dual fluid atomizer is commonly employed wherein an atomizing medium, usually a pressurized gas such as steam or air, is commingled with the liquid material within the atomizer and the mixture forced out under pressure. The expansion of the atomizing medium upon exiting the atomizer, along with the initial breaking up of the liquid material within the atomizer, provides the necessary energy to provide a good dispersion of the liquid material as a cloud of very fine droplets.
One common situation wherein a viscous material must be sprayed out as a cloud of fine droplets is in a combustion furnace utilizing heavy oil as fuel. The oil must be dispersed within the furnace in order to permit good mixing with the combustion air resulting in efficient and rapid combustion of the heavy oil. Such atomizers are common within the power generation industry and have been refined so as to perform satisfactorily over a wide range of sizes and fuel types.
Recently, interest in substantially liquid fuels consisting of a slurry of finely divided solids, such as coal, suspended in a carrier liquid, such as water, has been on the rise. Coal-water slurries, as these fuels are known, offer a considerable economic advantage over petroleum based fuels on a cost-per-BTU basis and may be applicable to a great number of situations in which the use of coal alone would be impractical.
While the technology exists to manufacture coal-water slurries combustible within a furnace, the development of an effective and reliable burner for these fuels has remained a stumbling block. One key element of any successful slurry fuel burner is an atomizer which is able to fragment the slurry fuel into readily combustible droplets or particles. The size, velocity and trajectory of these fuel droplets are functions of both the atomizer's design and the local aerodynamics, and directly effect overall burner performance in terms of flame length, stability and carbon burnout.
Two properties of coal-water slurry fuels have been identified as having the potential to cause problems in achieving effective atomization. These are the erosive nature of the slurry upon internal atomizer flow passages and the relatively high viscosity of coal-water slurry fuels relative to conventional petroleum products. The dual fluid atomizer discussed above has been shown to be effective for heavy oil and therefor seems a likely candidate for use with coal-water slurry fuels. Additionally, due to the simple geometry of the dual fluid atomizer design which has no tortuous paths, the atomizer may be easily fabricated with erosion resistant materials.
Attempts to design coal-water slurry atomizers based upon existing dual fluid atomizer technology has produced a number of workable, although compromise, designs. The peculiar nature of the coal-water slurry fuel has often restricted the designer from fully utilizing the inherent features of the dual fluid atomizer. By way of example, one method of increasing the dispersion of a liquid medium in a dual fluid atomizer is to increase the momentum of the liquid material as it is commingled with the atomizing medium. For a burner of fixed thermal output, the only manner of increasing the momentum of the liquid material is to restrict the liquid inlet passage. Although coal-water slurries generally behave much as a liquid, the slurry is still composed of particles suspended in a liquid carrier. Should the passage be restricted to a very small diameter, the larger particles present in the slurry may block the inlet passage and plug the atomizer. It is a good design practice to restrict the slurry passage diameter to no less than 10 times the diameter of the largest particle present in the slurry. Additionally, the coal-water slurry supply pumps generally available at this time are restricted to moderate outlet pressures in the range of 100 to 200 PSI (690 to 1380 kPa). This limits the total slurry pressure drop allowable across the atomizer and may as a result limit the mass flow through an otherwise adequately sized slurry passage. Higher pressure slurry pumps, while available from a limited number of manufacturers, are very costly.
Given these restrictions on the coal-water slurry supply pressure and inlet passage size, the remaining alternative left to the designer is to alter the momentum of the atomizing medium. By increasing the mass flow and/or decreasing the atomizing medium inlet passage, the designer can improve dispersion of the coal-water slurry at the cost of increased consumption of the atomizing medium and increased pumping costs resulting from the elevated pressure and flow.
Such designs, although workable, are still restricted in terms of performance. There is clearly a need for an efficient, low power consumption atomizer for coal-water slurry fuels or other viscous liquids that is resistant to plugging and able to deliver a spray of finely divided droplets with a minimum amount of atomizing medium for a given liquid flow.